1. Field of the Invention
The present invention relates to voltage sensing and transformer circuits.
2. Description of Related Art
Voltage sensing devices are used in various applications to measure voltages. Typical voltage sensing devices include a transformer connected on the primary winding to a voltage to be measured and connected on the secondary winding to a signal processing circuit. The voltage to be measured is transferred through the transformer to the signal processing circuit. The transformer and the signal processing circuit each have certain impedance characteristics. The impedance characteristics affect the voltage that is transferred through the transformer and the signal processing circuit. The voltage received at the signal processing circuit input is processed in light of the transformer and signal processing circuit impedance. The signal processing circuit outputs a value representing the value of the voltage measured. By knowing the impedance characteristics of both the transformer and signal processing circuit, it is possible to indicate a true value of the voltage measured at the output of the signal processing circuit.
Referring to FIG. 1, a typical circuit 100 for voltage sensing and related signal processing is shown. Circuit 100 includes transformer 110 coupled to a scaling circuit 160 comprising resistors 121, 122, and 123. Transformer 110 includes core 115, primary winding 111 and secondary winding 112. Primary winding 111 is coupled to receive a voltage to be sensed (V2, the voltage difference between terminals 11 and 12). As shown, secondary winding 112 is coupled at each respective end to, respectively, resistor 122 and resistor 123. Resistor 122 is coupled to resistor 121 and to voltage output terminal 13 as shown. Similarly, resistor 123 is coupled to resistor 121 and to voltage output terminal 14 as shown.
Voltage output terminals such as terminals 13 and 14 are typically connected to a signal processing circuit such as signal processing circuit 140. The temperature of the voltage sensing device will change due to any change in the ambient temperature of the surrounding environment. Also, when a voltage is transferred through a voltage sensing device, the temperature of the circuit 100 changes because the resistance to current flow of the material and other losses in these components creates heat. A change in temperature also produces a change in impedance in transformer 110 and a change in impedance of the scaling circuit 160.
The change in impedance of the transformer 110 is typically different than the change in impedance of the scaling circuit 160 because of the differences in material. Therefore, to accurately indicate the voltage being measured, the impedance characteristics of the transformer 110 and the scaling circuit 160 to which terminals 13 and 14 are connected, as well as the temperature of these components, must be known. Furthermore, the difference between the temperature dependence of the transformer 110 and the temperature dependence of the scaling circuit 160 produces an undesired temperature dependence of the overall circuit 150 (the transformer and scaling circuit combined). As the impedance of transformer 110 changes with a change in temperature, the voltage sensed in the signal processing circuit 140 also changes. This is because the signal processing circuit 140 indicates the voltage measured by processing the voltage Vo, present between nodes 13 and 14, which in turn is affected by the relationship of the impedance of the transformer circuit 110 and the impedance value of the scaling circuit 160. If the impedance value changes due to temperature changes, the voltage received, and therefore the voltage indicated, will be affected.
For example, at room temperature a voltage sensing device might read 100 volts when initially measuring a 100 volt source voltage. As the voltage continues to pass through the components of the voltage sensing and scaling circuit, the temperature changes causing a change in impedance of the overall circuit. As the impedance changes, the voltage received at the signal processing device will be different even though it is still the same 100 volt source being measured. When the voltage received at the signal processing circuit is different, the signal processing circuit will naturally output a different voltage and incorrectly indicate a different measured voltage. In our example, for instance, the voltage sensing device might read 102 volts when the temperature of the components change from room temperature. The change in the voltage measured by the same device as the temperature in the device changes is called voltage offset drift. Voltage offset drift is defined in terms of volts per degree of temperature increase and is caused by the change in impedance of the components in the voltage sensing device. Thus, the temperature dependence of the overall circuit in a voltage sensing device will produce a voltage offset drift that will affect the accuracy of the voltage measurement.
To combat voltage offset drift, conventional devices further process the voltage signal to remove the undesired temperature dependence of the overall circuit. These devices measure either the ambient or actual circuit temperature and then further process the signal to subtract out the offset drift based on the measured temperature. This further processing can be accomplished by summing the signal with a corrective signal to negate the error (offset drift) induced by the temperature change of the circuit and more accurately represent the actual measured voltage signal. However, this approach requires the measurement of the temperature and the use of further processing to arrive at an accurate voltage measurement. It also requires knowledge of the circuit's offset drift value (volts per degree of temperature rise).
However, it would be desirable to provide a voltage sensing circuit that does not require such extensive corrections as must be made with existing systems.